1. Field of the Invention
The invention relates to electrical distribution devices and, more particularly, electrical distribution devices, such as, for example, receptacle outlets, wiring devices, wall, light or other power switches, lamp bases, extension cord outlet boxes, or wire union junction boxes, having an arrangement for protecting against overheating conditions.
2. Description of the Prior Art
Electrical connections, especially where wires are terminated (e.g., at outlets, switches, or other electrical distribution devices), are susceptible to overheating conditions that can potentially cause a fire. The overheating conditions can be caused by a number of conditions such as a loose, damaged or degraded connection between an electrical conductor and a terminal. A loose, damaged or degraded connection in and of itself may not be a hazard, but it is known that such connections can cause arcs when current is flowing and/or cause fretting of the electrical conductor. The arcing and/or fretting can lead to problems that result in overheating conditions, such as a glowing contact.
A glowing contact is a high resistance connection which can form at the interface of, for example, a copper wire and a screw terminal of, for example, a receptacle. The high resistance connection results from a build up of copper oxide that is produced during arcing and/or fretting at the interface. During a glowing contact fault in, for example, a receptacle, the copper wire reaches a glowing temperature value at which time the wire looks like an electric heater coil. First, the wire's insulation melts at the terminal. The melting then slowly progresses away from the terminal toward other wires in the receptacle's outlet box. The melting and decomposition of the plastic insulation from the wire and outlet can produce ignitable gasses (e.g., hydrogen, methane, ethane, ethylene, or acetylene) which can be ignited by an arc. Plastics and surrounding materials (wood, wallboard, etc.) may also be ignited solely from the high temperature produced from the glowing connection.
Furthermore, the current that flows both during and after the formation of a glowing contact is typically normal, since the voltage drop across a glowing contact, depending on the current, can range from 2 Vrms to 10 Vrms, with the higher voltage level occurring at the lower current levels. The existence of a glowing contact, therefore, is not reliably detectable by a conventional upstream current protective device (e.g., a conventional circuit breaker or fuse).
It is thus desirable to be able to detect glowing contacts or other conditions that lead to overheating conditions and interrupt the current before the fault progresses to a hazardous condition.